In conventional third generation (3G) cellular systems, AM&C and HAR-Q techniques provide increased data rates and improved utilization of radio resources. Signaling between User Equipment (UE) and Base Stations (Node Bs) for the coordination of AM&C and H-ARQ functions is provided by physical control channels, such as High Speed Data Packet Access (HSDPA) control channels, or the like.
The number of bits allocated for a physical channel message is generally predetermined and restricted. As a result, an error detection scheme, such as a cyclic redundancy check (CRC), is not performed on a physical channel for either uplink or downlink signaling and is not available for use. A primary problem with ensuring successful transmission within the Medium Access Control (MAC) layer is the inability to detect and realize errors when they occur. When transmission errors occur on physical control channels in either the uplink or the downlink, such as with AM&C and H-ARQ control signaling, it is possible that transmission errors are not perceived by the physical layer (i.e. existing MAC layer data recovery mechanisms). Therefore, these MAC layer data recovery mechanisms are not initiated.
Accordingly, current 3G systems are not adequately configured to cope with the occurrence of errors on physical control channels. This results in a high probability of UE to Node B H-ARQ acknowledgements and channel quality indications for determination of AM&C to be misinterpreted, which further results in a loss of data within the MAC layer and inefficient use of radio resources.
When a loss of data occurs in the MAC layer, the Radio Link Control (RLC) layer initiates mechanisms to recover the lost data. One disadvantage of relying on the RLC layer to recover the data is the latency of transmission, since the round-trip delay of the retransmission is significantly longer than that of transmissions at the H-ARQ level. This results in large data buffering requirements in the UE and a reduction in data throughput.
Accordingly, for efficient use of radio resources, for a higher user Quality of Service (QoS) and for the MAC to successfully recover failed transmissions, it is necessary to maintain low error rates on the AM&C and H-ARQ control channels. The current use of physical channel control fields for AM&C and H-ARQ signaling severely limits the ability to maintain low error rates.
One example of a type of failure which is not detected during the physical control signaling is the misinterpretation of generated acknowledgement (ACK) messages and negative acknowledgement (NACK) messages. An ACK message indicates a successful transmission of a data block, while a NACK message indicates a failed transmission of a data block.
FIG. 1 illustrates a prior art system 100 in which a transmitter 105 generates and sends a data block 110 comprised of one or more protocol data units (PDU) to a receiver 115. The data block 110 is not received correctly by the receiver 115. Although, the receiver 115 generates a NACK message 120, this NACK message 120 is corrupted during transmission and is interpreted by the transmitter 105 as being an ACK message 122. Thus, the transmitter 105 is never made aware that a transmission failure has occurred, and continues with another transmission without recovering and retransmitting the failed data block 110. In this case of a NACK being misinterpreted as an ACK, recovery by the RLC is required. However, as aforementioned, the RLC takes a considerable amount of time to recover the data, which is undesirable.
FIG. 2 illustrates a prior art system 200 in which a transmitter 205 generates and sends a data block 210A to a receiver 215. The data block 210A is received correctly by the receiver 215. Although, the receiver 215 generates an ACK message 220, this ACK message 220 is corrupted during transmission and is interpreted by the transmitter 205 as being a NACK message. Thus, the transmitter 205 interprets that a failure has occurred and unnecessarily retransmits the data block 210B. In this case of an ACK being misinterpreted as a NACK, a failure is incorrectly indicated and an unnecessary retransmission is generated. This is an inefficient use of radio resources.
FIG. 3 illustrates a prior art system 300 in which a transmitter 305 generates and sends an allocation message 310 to a receiver 315, which is not received by the receiver 315 due to the great amount of corruption and/or interference. As a result, the receiver 315 fails to send a message back to the transmitter 305. In the interim, the transmitter 305 waits to receive a message back from the receiver 315, (i.e., an ACK or NACK message). The transmitter 305 is unaware that the receiver 315 did not receive the allocation message 310. Because the transmitter 315 is waiting to detect a message that never arrives, due to the great amount of corruption there is a high possibility that bit estimation and/or power threshold errors will cause noise 320 to be incorrectly be detected by transmitter 305 as being an ACK message, even though a failure exists.
Another example of a type of failure which is not detected during the physical control signaling is the misinterpretation of the channel quality indicator (CQI) which is sent as part of the AM&C process, as shown in the system 400 of FIG. 4. The receiver 415 receives a plurality of PDUs 410 and generates a CQI as an indication of the quality of the data being received by the receiver 415. The particular mechanism used to generate the CQI is outside the scope of the present invention and, accordingly, will not be described in detail herein. However, after the CQI 420 is generated it is transmitted from the receiver to the transmitter 405, which utilizes the CQI during the AM&C process to select the proper modulation and coding set (MCS).
As those of skill in the art would appreciate, if the CQI 420 is corrupted during the transmission of the CQI 420 from the receiver 415 to the transmitter 405, an incorrect or corrupted CQI 422 will be received by the transmitter 405 and utilized during the AM&C process. If the corrupted CQI 422 is incorrectly low, (erroneously indicating that channel conditions are poor), a more robust MCS than is necessary will be selected. This results in a waste of radio resources. Alternatively, if the corrupted CQI 422 is incorrectly high, (erroneously indicating that channel conditions are favorable), a less robust MCS than needed will be selected, subjecting the data transmissions to a high error rate.
These two types of failures, (i.e. H-ARQ signaling and AM&C signaling), illustrate several of the many signaling errors that may occur between the Node B and a UE.
In 3G cellular systems, a wide range of services are provided; from high data rate services such as video and Internet downloads, to low data rate services such as speech. Referring to FIG. 5, a plurality of user services is shown as individual data streams. These, individual data streams (i.e., user services) are assigned to respective transport channels A,B,C whereby the data streams are coded and multiplexed. Each respective transport channel A, B, C is assigned a specific coding rate and a specific transmission time interval (TTI). The coding rate determines the number of transmitted bits of the physical layer and the TTI defines the delivery period of the block of data to be transmitted. For example, the TTI may be, for example, 10, 20, 40 or 80 ms.
Multiple transport channels A, B, C are multiplexed together into a coded composite transport channel (CCTrCh) consisting of a common set of physical channels. Since the CCTrCh is made up of a plurality of transport channels A, B, C it may have a plurality of different coding rates and different TTIs.
For example, a first transport channel A may have a 20 ms TTI and a second transport channel B may have a 40 ms TTI. Accordingly, the formatting of the first transport channel A in the first 20 ms and the formatting of first transport channel A in the second 20 ms can change. In contrast, since the second transport channel B has a 40 ms TTI, the formatting, and hence the number of bits, are the same for each 20 ms period over the 40 ms TTI duration. It is important to note that all of the transport channels A, B, C are mapped to the CCTrCh on a TTI basis, using the smallest TTI within the CCTrCh.
Each individual data stream has an associated data rate, and each physical channel has an associated data rate. Although these data rates are related to each other, they are distinctly different data rates.
Once the smallest TTI within the CCTrCh has been established, it must be determined how many bits of data will be transmitted and which transport channels will be supported within a given TTI. This is determined by the formatting of the data. A Transport Format Combination (TFC) is applied to each CCTrCh based on the smallest TTI. This essentially specifies for each transport channel how much data is transmitted in a given TTI and which transport channels will coexist in the TTI.
A TFC Set (TFCS) is the set of all of the possible TFCs. If the propagation conditions do not permit all of the possible TFCs within the TFCS to be supported by the UE, a reduced set of TFCs which are supported by the UE is created. This reduced set is called a TFC subset. TFC selection is the process used to determine which data and how much data for each transport channel to map to the CCTrCh. A transport format combination indicator (TFCI) is an indicator of a particular TFC, and is transmitted to the receiver to inform the receiver which transport channels are active for the current frame. The receiver, based on the reception of the TFCIs, will be able to interpret which physical channels and which timeslots have been used. Accordingly, the TFCI is the vehicle which provides coordination between the transmitter and the receiver such that the receiver knows which physical transport channels have been used.
It would be desirable to minimize the need for RLC data recovery by maintaining low error rates on AM&C and H-ARQ control channels, rather than physical channels.